The most concentrated area of ​​the automotive chip market: SerDes introduction

SerDes stands for serialization and deserialization. In the automotive field, each camera requires at least one serializer and at least 0.25 deserializers. Each display requires a serialization and a deserialization chip. The global market size in 2023 is about 2.5-3 billion US dollars. Although the market size is not large, it is growing strongly. The driving reason is self-evident, especially in the domestic automotive industry, which is seriously involuted. The number and pixels of cameras are increasing rapidly, and the number and resolution of displays are also increasing rapidly. In the field of ADAS and 360 panoramic cameras with more than 4 million pixels, ADI (mainly derived from the product line of Maxim acquired for US$20.9 billion in 2021) completely monopolizes the market. In the 360 ​​panoramic market with less than 4 million pixels, ADI also completely monopolizes. Only in the ADAS market, Texas Instruments has a small share. In the display field, Texas Instruments accounts for about 70% of the market, and ADI's share is very low. In the display field, there is also Sony's GVIF, which is mainly used in Japanese cars. Japanese car automotive electronics lags far behind the world, especially China, so Sony GVIF has a very low market share. There is also Inova's APIX3, which basically has only one customer, BMW. Valens basically only has Mercedes-Benz as a customer.

Application of SerDes in the automotive field

Image source: ADI

SerDes is not only widely used in the automotive field, but also in robotics, medical, and video acquisition fields. It is also widely used in the server field. The server field is more valuable, and the chip price is as high as thousands or even tens of thousands of dollars, which is basically monopolized by Broadcom.

信号传输分为并行与串行两种，在高速状态下，并行口的几根数据线之间存在串扰，而并行口需要信号同时发送同时接收，任何一根数据线的延迟都会引起问题。而串行只有一根数据线，不存在信号线之间的串扰，而且串行还可以采用低压差分信号，可以显著提高它的抗干扰性，因此可以实现更高的传输速率。尽管并行可以一次传多个数据位，但时钟频率远远低于串行，此外串行的传输介质通常是同轴电缆，连接器是FARKA，并行的传输介质和连接器成本都高于串行，所以目前串行传输是高速传输的首选。

SerDes reshapes the data format into serial before transmission and converts the data format into parallel after reception. It has two chips, the transmitting end is called Serializer and the receiving end is called Deserializer. SerDes does not transmit clock signals, which is also the most special feature of SerDes. SerDes integrates CDR (Clock Data Recovery) circuit at the receiving end, uses CDR to extract the clock from the edge information of the data and find the optimal sampling position. This is also the most difficult part, because it transmits digital signals, but the technology required is analog technology.

CDR, or clock data recovery, is a circuit for high-speed serial buses. In general, clock information is embedded into the transmitted data stream through data encoding, and then the clock information is extracted through clock recovery at the receiving end, and the data is sampled with the recovered clock. Therefore, the clock recovery circuit is crucial for the transmission and reception of high-speed serial signals. The main design challenge of the CDR interface is jitter, that is, the offset of the actual data transmission position relative to the expected position. Total jitter (TJ) consists of deterministic jitter and random jitter. Most jitter is deterministic, and its components include inter-symbol interference, crosstalk, duty distortion, and periodic jitter (for example, interference from switching power supplies). Random jitter is usually a byproduct of semiconductor heating problems and is difficult to predict. Transmit reference clocks, transmit PLLs, serializers, and high-speed output buffers all affect transmit jitter. Generally speaking, the tolerance for low-frequency jitter is very high, and the PLL circuit can track it well, and the recovered clock jitters with the measured signal. High frequencies are more troublesome, as you have to set up a PLL circuit to filter them out. How to set it up depends entirely on experience without computer assistance, and it cannot be done well without about 10 years of experience.

This also makes the moat of interface IC very wide, allowing the existence of very small manufacturers, which may only have one product but are extremely tenacious. Typical examples are Inova, which basically has only one customer, BMW, and Valens, which basically has only one customer, Mercedes-Benz.

In the camera field, GMSL occupies a monopoly in the high-pixel field. GMSL (Gigabit Multi-Media Serial Link) is ADI's serial deserialization technology.

Image source: ADI

ADI or Maxim's GMSL technology first appeared in 1999, and the first product was launched in 2004. Currently, the third-generation GMSL has been completed and the GMSL3X under development can be regarded as the 3.5-generation GMSL.

Performance comparison of four generations of GMSL

Image source: ADI

There are two versions of the first generation of GMSL, the advanced version shall prevail.

Image source: ADI

Currently, GMSL2 is the main standard on the market, while GMSL3 can support up to 15 million pixels. Basically, GMSL2/3 product datasheets need to sign NDA, and it is difficult for the outside world to obtain relevant information. GMSL4 seems to want to compete with traditional PCIe/USB and step out of the automotive field.

GMSL needs to support the use of coaxial and twisted pair cables. The high-frequency attenuation of low-cost twisted pair cables is a serious problem. High-frequency attenuation causes significant inter-symbol interference (ISI) in the received signal, making it difficult to recover the clock and data, resulting in an increase in the bit error rate (BER). The transmitter and receiver adopt a certain form of line equalization to significantly reduce ISI and recover severely degraded data to ensure reliable operation.

What really affects the reliable transmission of signals is not the attenuation itself, but the change of channel attenuation with frequency. The difference in attenuation between high-frequency and low-frequency signals will eventually lead to inter-symbol interference (ISI). Literally, inter-symbol interference is the interference between different code elements. For example, the "1" signal transmitted at time A is superimposed on the "0" signal transmitted at time B, causing the signal amplitude at time B to change from 0 to 0.2. Why does the difference in attenuation between high-frequency and low-frequency signals lead to inter-symbol interference? Because the loss of the high-frequency component of the signal will slow down the signal edge, resulting in signal broadening. The broadened signal may span multiple unit time intervals (1UI), and the signal at time A mentioned above will be superimposed on time B. The greater the attenuation of the channel, the more serious the signal broadening, and the greater the proportion of superposition on the signal at other times. In other words, what the SerDes system needs to solve is not the attenuation of the signal, but the difference in attenuation between high-frequency and low-frequency signals. The solution is mainly to add weight and balance, which depends entirely on rich experience accumulation. Only Texas Instruments and ADI can do it in the world. Broadcom should be able to do it, but it seems to lack interest in the automotive field. SerDes for servers is more profitable.

GMSL 1 is fixed equalization, 2 is adaptive equalization AEQ, and 3 adds Feed Forward Equalizer (FFE). The adaptive equalizer enables GMSL2 links to resist noise, crosstalk, and reflections. The equalizer amplifies high-frequency signals, and when combined with the frequency response of the cable, the receiver can recover broadband signals with higher fidelity. The equalizer has 12 different compensation levels. The chip automatically detects the quality of the input signal and adaptively sets the optimal equalization value, enabling the SerDes system to handle up to 30m of coaxial cable and 15m of STP cable length. Adaptive compensation is performed every time the deserializer is powered on, and it is called at a rate of approximately 1Hz to track temperature and voltage changes. Therefore, even if there are actual differences such as aging, temperature drift, and individual differences in the harness, AEQ can automatically select the best compensation level. In addition, technicians can also read the compensation value of AEQ after power-on. If it is significantly higher than the normal value, it can be judged that the current transmission channel may have abnormal conditions such as short circuit, looseness, and bending.

Pre-emphasis is a signal processing method that compensates for the high-frequency components of the input signal at the transmitting end. As the signal rate increases, the signal is greatly damaged during the transmission process. In order to obtain a better signal waveform at the receiving terminal, it is necessary to compensate for the damaged signal. The idea of ​​pre-emphasis technology is to enhance the high-frequency components of the signal at the beginning of the transmission line to compensate for the excessive attenuation of the high-frequency components during the transmission process.

The GMSL2 link includes an echo cancellation circuit in both the serializer and deserializer to achieve simultaneous transmission of high-speed video data and bidirectional control data. In fact, this technology is also used in automotive Ethernet, with the same principle, to achieve bidirectional data transmission on a single channel.

At present, the serializers are mainly MAX9295 and MAX96717. In terms of deserializers, in the field of 360-degree surround view, MAX96712 can correspond to four 4-megapixel cameras, which is the most mainstream product at present. Four 2-megapixel cameras are mainly MAX96722/MAX96724/MAX96716F. Currently, the GMSL3 generation products are mainly MAX96792/MAX96793, which can correspond to four 8-megapixel cameras with a rate of up to 12Gbps. At present, the highest product of Texas Instruments is DS90UB9702, with a Line rate of 7.55Gbps and a maximum rate of 10Gbps.